Field of the Invention
The present invention relates to an ink for manufacturing a display device using an inkjet method and a method for manufacturing the display device by using the same.
Discussion of the Related Art
Recently a flat panel display (FPD) has become important in accordance with the development of multimedia. Accordingly, various display devices such as a liquid crystal display (LCD) device, a plasma display panel (PDP), a field emission display (FED) device, an organic light emitting diode (OLED) display, and the like have been put to practical use.
Among the display devices, the organic light emitting diode (OLED) display device is a self-light emitting element that emits light by itself while electron-hole pairs disappear after electrons and holes are paired when the charges are injected into an organic light emitting layer formed between a cathode which is an electron injection electrode and an anode which is a hole injection electrode. In the organic light emitting diode display device it is possible to form the element on a flexible substrate such as a plastic and to drive at a low voltage of 10V or less than the plasma display panel (PDP) and an inorganic light emitting diode display. Therefore, it has the advantages in that the power consumption thereof is relatively small and the color quality is excellent. In addition, the organic light emitting diode display device may represent the three colors red, green, and blue, such that the organic light emitting diode display device is the subject of interest of many people as the next generation display representing rich colors.
In order to secure competitiveness of the organic light emitting diode display device, the manufacturing process needs to be enlarged. An increase in a size of the substrate to be inserted thereon may achieve high productivity and diversification of the products that are produced. To this end, the enlargement of the manufacturing equipment and the development of process technology and materials are needed. In order to increase the area of the substrate in a vacuum deposition process, many improvements are needed in a source of metal or organic matter, a metal mask, a glass substrate and a mask transport system, a construction of the equipment. The organic electro-luminescence element based on low molecular weight is possible to stack in layer by layer deposited through the vacuum deposition process. In order to pattern the R, G, and B sub-pixels, the mask is needed, but the size of a fine metal mask (FMM) increases as the area of the substrate increases, such that the problem of dropping the mask may occur. On the other hand, a solution process is possible to perform the pattern coating over the large area without using the mask through an inkjet printing or a nozzle printing, and utilization of the material of the solution process is very high and about 50 to 80% compared to the vacuum deposition process that utilization of the material is 10% or less. In addition, a glass transition temperature is higher than a thin film obtained by the vacuum deposition process, such that the solution process has thermal stability and excellent morphology characteristics.
As a representative method of the above-mentioned solution process, there is a method using the inkjet device. An ink used in the inkjet device is formed by mixing one solvent and a light emitting solid material. However, after the ink drops onto the pixel, a coffee ring and pile-up phenomena occurs during the thinning process according to drying, such that the thickness of the thin film becomes non-uniform and when the light emits, the electric conductivity becomes non-uniform, such that a luminance becomes non-uniform. FIG. 1 is a schematic view of the coffee ring phenomenon of the ink and FIG. 2 is a schematic view of a volume decrease phenomenon of the ink. Referring to FIG. 1, the ink is beginning to dry from the edge portion due to the difference in the evaporation rate of a central portion of the ink drop in which volume per unit area is small and an edge portion which volume per unit area is large. Therefore, the coffee ring phenomenon is caused by a capillary flow phenomenon that the ink flows from the central portion to the edge portion due to the difference in distribution of solids in the central portion and the edge portion according to drying of the ink. In addition, referring to FIG. 2, when drying the dropped ink, the ink dries gradually from the edge portion thereof to the central portion thereof. Here, the volume of the ink gradually decreases due to evaporation as it moves from the edge portion to the central portion. The precipitation of the solids of the ink due to the evaporation of the solvent according to the drying of the ink is carried out from a saturation region section of the solvent. That is, since the solids due to the evaporation at a portion in which the solids are unsaturated are not precipitated, the precipitation of the solids are not uniform according to the saturation of the ink solvent in all sections the ink is spread, such that the thickness of the ink thin film becomes non-uniform.
Therefore, due to the problems that occur during the drying process of the ink as described above, the light-emitting property of the organic light emitting diode display device deteriorates.